A metronome is a piece of laboratory apparatus and is a device that produces regular ticks (beats). More precisely it produces a regulated aural, visual or tactile pulse. It dates back to the early 19th century. A metronome is used by some performing musicians for practice in maintaining a consistent tempo; it gives composers an approximate way of specifying the tempo Many consider a metronome as an overly restrictive way of specifying tempo, since it cannot account for accellerando, rallendando, rubato, rhythmic alteration, spontaneous creative choices in rhythmic nuance etc. Ultimately the timing-details of an expressive performance cannot be notated with metronome markings.

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Metronomes may be used by musicians when practicing in order to maintain a constant tempo; by adjusting the metronome, facility can be achieved at varying tempi. Even in pieces that do not require a strictly constant tempo (such as in the case of rubato), a metronome "marking" is sometimes given by the composer to give an indication of the general tempo intended, found in the score at the beginning of a piece or movement thereof.

Tempo is most always measured in beats per minute (BPM); metronomes can be set to variable tempi, usually ranging from 40 to 208 BPM. Although rare, another marking denoting metronome tempi is M.M., or Mälzel's Metronome.

One common type of metronome is the mechanical metronome which uses an adjustable weight on the end of an inverted pendulum rod to control the tempo: The weight is slid up the pendulum rod to decrease tempo, or down to increase tempo. (The mechanism is also known as a double-weighted pendulum. There is a second, fixed weight on the other side of the pendulum pivot, hidden in the metronome case.) The pendulum swings back and forth in tempo, while a mechanism inside the metronome produce a clicking sound with each oscillation.

Most modern metronomes are electronic and use a quartz crystal to maintain accuracy, comparable to those used in wristwatches. The simplest electronic metronomes have a dial or buttons to control the tempo; some also produce tuning notes, usually around the range of A440 (440 hertz). Sophisticated metronomes can produce two or more distinct sounds. Tones can differ in pitch, volume, and/or timbre to demarcate downbeats from other beats, as well as compound and complextime signatures.

The clicking sounds of mechanical metronomes have been sometimes used to provide a soft rhythm track without using any percussion. Paul McCartney did this twice: in 1968 on "Blackbird", included on the famous "White Album" The Beatles, and in 1989 on "Distractions" (Flowers in the Dirt) , where McCartney, following the metronome's regular beat, performed the whole rhythm track by hitting various parts of his own body.[2] Also, in Ennio Morricone's theme "Farewell to Cheyenne" (featured on Once Upon a Time in the West), the steady clip-clop beat is provided by the deliberately distorted and slowed-down sound of a mechanical metronome.[3]